• Created by Unknown User (cpoler@mit.edu), last modified by Unknown User (vandeamy@mit.edu) on Apr 01, 2017 01:27

To make a composite tube, one must use a mandrel, which holds the fibers and resin in place until they are cured. Structures has used several different mandrels in the past couple of years.

Steel tube

The steel mandrel was a 6" OD, 60" long, 1/8" thick steel tube. This mandrel was notably quite round without any machining, but it was extremely difficult to remove the cured parts from the mandrel. We think the steel was a structural steel tube since it has a weld.

Aluminum tube

To remove parts more easily, we tried using aluminum instead, because we hoped that cooling the aluminum mandrel would cause it to shrink. However, a fiberglass composite has roughly the same coefficient of thermal expansion as aluminum, so this was ineffective.

The first of the aluminum mandrels was a 6" OD, 60" long, 1/8" thick aluminum tube. However, the mandrel was about 0.125" out of round (i.e. roughly an ellipse, where the major axis was about 0.125" longer than the minor axis). We hypothesize that this is because the mandrel was not shipped well, and was likely crushed during shipping.

The second of the aluminum mandrels was a 6" OD, 60" long, 1/4" thick aluminum tube. This mandrel was out of round by about 0.030", seemingly due to the aluminum extrusion process.

Bidirectionally tapered tubes

Again, to remove parts more easily, we tried a tapered mandrel. We used two 30" long aluminum tubes to create a mandrel where the ends were 6.00" OD, and the center was 5.94" OD, again 60" long with a roughly 1/4" wall thickness. To keep them together, we epoxied a coupler section to the middle with Proline. The tubes were fairly easy to remove from this mandrel.

Note: The mandrel must be supported at both ends for machining, so we used an endplate with a hole for the tailstock. Also, when machining these parts, the lathe can create an incredible amount of chatter, which causes poor surface finish and a deafening noise. To dampen such vibrations, we inserted MDF disks inside the mandrel. The parts were machined in the Hobby Shop using a CNC program for the taper.

 

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